1. Field of the Invention
This invention relates generally to clocks, and more particularly to techniques for guaranteeing the accurate synchronization of clocks. It also relates to telephony applications based on Caller ID delivery.
2. Brief Description of Prior Art
Clocks have been around since the first sundial, and the problem of making them report time accurately has been a continuing struggle for all of recorded history. Pendulum, flywheel and spring driven clocks are of modest accuracy, requiring manual correction anywhere from monthly to daily if one intends to report time accurate to a couple of minutes. Nonetheless they are sufficiently accurate for average human needs and many of them still serve today. The advent of modern electrical AC power, with its highly accurate alternations, led to clocks that rarely needed correction more than once a month, and they could generally report time accurate to the minute. Corrections for this type of clock are usually still performed manually, and as a result a clock may be inaccurate by up to several minutes over prolonged periods of time without being noticed and corrected. Interruptions in electric power supply, which happen virtually everywhere in the world from time to time, then demand correction. Self-contained clocks synchronized by crystal oscillation, such as modern battery-powered watches or the timers in computers, vary as little as a few seconds per month, and hence need only very infrequent resetting. These generally also rely on a manual correction technique, and thus on the notice and control of a human being.
The above types of clocks probably comprise the majority of clocks currently in use in the world today. Timekeeping in certain applications requires greater regularity than common clocks deliver. Institutions which run on an exacting schedule, such as large factories, schools, railway systems etc., often have clock systems where individual clocks are all connected to a correction circuit driven by a central clock which delivers a pulse once per minute, advancing all clocks with the same pulse. This system guarantees agreement among all clocks nearly to the second, and manual intervention is required only after a system-down event. The central clock is declared the "official" time. A particular example of such institutions, pertinent to the current invention, is the local telephone service provider, which for billing and other reasons maintains a central clock tightly synchronized with legal official time. Recently such providers have been required to offer a service known as "Caller ID" throughout the United States and Canada. The Caller ID is an information packet sent over the telephone transmission route which contains (among other things) the calling number and the date and time of the call.
Radio-controlled clock systems allow extremely accurate synchronization of clocks over wide distances of separation, especially where the clocks are not stationary (ships and aircraft, for example), or where it is difficult or impossible to lay electrical wiring infrastructure. Conklin et al., U.S. Pat. No. 4,823,328, demonstrates such a clock system, and Cateora et al., U.S. Pat. No. 4,014,166 even uses a satellite as a broadcast source. Here the "official" time is the controlling clock at the broadcast source, which might even be an atomic clock at a standards institution. Such broadcast clock systems are very costly to operate, and the receiving clock stations are sophisticated and costly devices. With them, however, manual intervention for setting is no longer required.
It is an object of this invention to create a clock which can be produced and operated at a low cost, which is accurate to standard official time within a tolerance sufficient for everyday usage, but without the need for manual setting intervention.